This invention relates to an automatic gain control apparatus, including a servo control unit responsive to a servo error signal for locating a movable head with respect to a recording medium.
Optical disks have been employed as a recording medium has been employed to store data on tracks thereof. An optical head is used to play back the data recorded on the optical disk. A typical optical head includes a light source, such as a semiconductor laser, for radiating a light (laser) beam through an objective lens onto the optical disk. The objective lens is positioned at a predetermined distance from the optical disk for focusing the light beam on the data recording layer on the optical disk. The optical head also includes 40 photo sensors for sensing the light beam reflected from the optical disk to determine the presence and absence of pits formed in the spiral tracks of the optical disk. The objective lens is supported in such a manner that its optical axis can follow the spiral track and can move from one track to a desired track during a seeking operation, as well as to the direction of the optical axis for focusing.
In order to locate the optical head at an optimum position against the optical disk, the optical head is associated with a focusing servo unit for moving the objective lens in a focusing direction parallel to the optical axis thereof so as to maintain the predetermined distance from the optical disk when the distance of the objective lens from the optical disk deviates from the predetermined distance. The optical head is also associated with a tracking servo unit for moving the objective lens in a tracking direction perpendicular to the track to locate the light spot on the optical disk at the center of the track when the optical axis deviates from the track center.
A part of the light beam reflected on the optical disk is incident on a photo sensor divided into four independent elements A1, B1, C1 and D1, as shown in FIG. 1A. Another part of the reflected light beam is split into two light beams incident on respective two photo sensor elements E2 and F2, located at positions corresponding to the opposite sides of the track at the same distance from the track center, as shown in FIG. 1B. Each photo sensor element converts the light incident thereon into an electric signal having a level corresponding to the intensity of the incident light beam. The electric signals derived from the photo sensor elements A1 and C1 are added to form an FA signal. The electric signals derived from the photo sensor elements B1 and D1 are added to form an FB signal. The photo sensor element E2 converts the light incident thereon into a TA signal having a level corresponding to the intensity of the incident light, and the photo sensor element F2 converts the light incident thereon into a TB signal having a level corresponding to the incident light. The playback signals derived from the photo sensor elements A1 to D1, E2 and F2 are added to form an RF signal. The RF signal is to read data recorded on the optical disk but the TA and TB signals are to generate a tracking error signal and the latter is not required to include data component recorded on the optical disk. Therefore the TA and TB signals is limited to a narrow band of frequencies (e.g., below 50 KHz).
Each track comprises a plurality of sectors, each having a pre-format area preceding to a user area on which pits are formed to store data from, for example, a music or image source. The pre-format area has pre-format data previously recorded thereon for use in recording and playing back data on the user area.
FIG. 2A shows a waveform of a pre-format signal included in the RF signal resulting from playback of the pre-format data recorded on the pre-format area under a tracking servo control. The pre-format signal includes a sector mark (SM), a mirror mark (ODF) and VFO and ID signals repeated three times alternatively between the sector mark (SM) and the mirror mark (ODF). The sector mark indicates the start of the pre-format signal. Each VFO signal contains clock pules required to reproduce the succeeding ID signal and has a frequency higher than any other signals recorded on the optical disk. Each ID signal includes at least a sector address and an error detection code. The mirror mark (ODF) is used to adjust the electrical offset of the tracking servo circuit. The mirror mark (ODF) has a level higher than any other signals recorded on the optical disk and a signal from a non-recorded area on the track which is formed as a pre-groove because the ODF is formed by not forming the pre-groove, namely by leaving a mirror surface of the optical disk.
FIG. 2B shows the RF signal on a reduced time scale. As shown in FIG. 2C, the TA (or TB) signal, which is produced from the photo sensor element E2 (orF2), has a level that is lower when the optical head passes the pre-format area because the pre-format data is recorded on the pre-format area. However, the data recorded on the optical disk can not be played back from the TA (or TB) signal.
FIG. 2D shows the RF signal on a further reduced time scale. It is assumed that the user area of the data sector No. N+1 has recorded data thereon, as indicated by the hatched area in FIG. 2D. As shown in FIG. 2E, the level of the TA (or TB) signal is also lower at the user area of the data sector No. N+1, as well as the pre-format area. It is, therefore, apparent that the level of the tracking error signal produced from the difference (TB-TA) between the TB and TA signals is reduced to a smaller level in the pre-format and user areas having data recorded thereon than in the user areas having no data recorded thereon. Consequently, the tracking servo control is particularly sensitive to disturbances when the optical head passes the recorded areas.
In order to avoid the difficulty, an automatic gain control (AGC) circuit is required to compensate for the level drop of the tracking error signal in the recorded areas. In the previous apparatus, the gain control is performed on an assumption that the level drop of the tracking error signal can be represented merely by the corresponding level drop in the RF signal. Therefore, it was very difficult to provide an accurate compensation for the level drop of the tracking error signal because track cross components are superimposed in the (TA+TB) signal and the tracking error signal (TA-TB), as shown in FIGS. 2F and 2G, respectively, when the optical head traverses the tracks with the tracking servo control being suspended.
Such a gain control will render it difficult to ensure a rapid lock-in operation when the tracking servo control is resumed. In addition, the counter, which is used to count the number of peaks of the RF signals so as to count the number of traversed tracks, would accumulate an incorrect count.
Furthermore, since a power of the laser beam incident on the optical disk is increased in a recording mode in comparison with a playback mode, the output level of the photo sensor is also increased. In a conventional apparatus, such a level increase at the time of recording is also controlled by the automatic gain control circuit. Therefore, a very wide dynamic range is required in the automatic gain control circuit to control both of the lower level at the time of playback and the higher level at the time of recording. In addition, when the mode is switched from the recording mode to the playback mode, the tracking servo tends to become unstable, so that the lock of the tracking servo is apt to be removed by the disturbances.